Loss of superfluidity by fermions in the boson Hubbard model on an optical lattice

TL;DR

This paper explains how fermions in a boson Hubbard model on an optical lattice cause a loss of superfluidity by increasing on-site repulsion through virtual transitions and interactions, affecting the phase diagram.

Contribution

It introduces a mechanism where fermions enhance the Mott insulating phase by increasing boson on-site repulsion via virtual band transitions and contact interactions.

Findings

Fermions can suppress superfluidity in bosonic systems.

The Mott lobes are expanded due to fermion-induced renormalization.

Implications for cold atom experiments are discussed.

Abstract

The experimentally observed loss of superfluidity by introducing fermions to the boson Hubbard system on an optical lattice is explained. We show that the virtual transitions of the bosons to the higher Bloch bands, coupled with the contact boson-fermion interactions of either sign, result in an effective increase of the boson on-site repulsion. If this renormalization of the on-site potential is dominant over the fermion screening of the boson interactions, the Mott insulating lobes of the Bose-Hubbard phase diagram will be enhanced for either sign of the boson-fermion interactions. We discuss implications for cold atom experiments where the expansion of the Mott lobes by fermions has been conclusively established.